1. Field of the Invention
This invention relates to water-soluble, polyurethane comb polymers effective in thickening aqueous systems.
2. Description of the Prior Art
There are several basic theories by which water-soluble polymers are believed to thicken aqueous systems such as waterborne coatings and latex paints. The theory of "Chain Entanglement" requires the polymer to have a very high molecular weight which in solution translates to an extremely large hydrodynamic volume. The thickening occurs because the long, solvated polymer chains comingle with one another producing "chain entanglements." The salient points of this model are (a) there is no interaction of the water soluble thickener polymer chains with the latex particles; (b) under shear conditions, the water-soluble thickener polymer chains orient or are visco-elastically deformed resulting in a lowered viscosity (shear thinning); and (c) upon removal of the shear the visco-elastic polymer chains recover immediately resulting in very poor flow and leveling properties for the aqueous system. It is believed that the more established thickeners, i.e., the traditional cellulosics, natural gums, and very high molecular weight, synthetic, water-soluble polymers achieve their thickening effects by this "chain entanglement" mechanism.
Another theory of thickening in aqueous systems can be called "Particle Bridging" or "Association Thickening." This theory is proposed by coating technologists of the Dow Chemical Company to explain the thickening action which occurs in latexes and waterborne coatings by certain synthetic thickeners. The Particle Bridging theory is described in a pamphlet entitled "ELT Experimental Liquid Thickeners XD-30255.02L and XD-30457.02L" distributed by the Dow Chemical Company.
The Particle Bridging theory is offered to account for the thickening properties of relatively low molecular weight synthetic thickeners described in a series of patents including U.S. Pat. No. 3,779,970 (Evani I) that describe polymers which are composed of two different segments: (1) the backbone of the polymer, which is water-soluble, and (2) long polyalkylene oxide branches each terminated by a hydrophobic moiety. The long branches are attached to the polymer backbone much like the teeth of a comb to the comb's backbone, such that these polymers are characterized as "comb polymers". The combination of the polyalkylene oxide branches terminated by the hydrophobic moieties provides the polymer with certain qualities of a surfactant. Consequently, the thickening action of these comb polymers is suggested to be by particle-to-particle bridging in which the hydrophobic moiety terminating the individual teeth of the copolymer is postulated to adsorb at the latex particle surface much in the nature of a surfactant. Since there is a plurality of teeth on each polymer backbone, simultaneous interaction of a single polymer molecule with two or more particles can create an apparent three-dimensional network. This pseudo-network is considered to account for a viscosity increase. The important aspects of the particle bridging theory are (a) a specific interaction, i.e., adsorption, is required of the surfactant tooth at the particle surface; being that, it is assumed that the hydrophobic tooth moiety displaces the stabilizing surfactant or colloid already present on the surface of the particle; (b) under a shear field or shear inducement, the bridged particles of the latex are mechanically separated causing the adsorbed tooth-like moieties to be wrested from the surface, i.e., desorbed such that there is a viscosity decrease (shear thinning); and (c) upon removal of the shear force, the recovery is diffusion controlled and governed by the rate of readsorption resulting in a viscosity increase at a relatively controlled rate so as to achieve good flow and leveling properties. The polymers of these structures are indicated to exhibit a better rheology than those which operate by the "chain entanglement" mechanism. Such polymers are alleged to provide much better flow and leveling characteristics to waterborne coatings and latex systems than do the traditional cellulosic thickening agents.
In relying upon the Particle Bridging theory one has to stress the criticality of the total polymeric molecular structure because in order to have the hydrophobic moiety adsorb on the latex particle surface, the hydrophobic moiety must be chemically attached to a hydrophilic moiety of a type which allows particle adsorption and displacement of the surface active agent or colloid which is already present on the particle surface. In particular, U.S. Pat. No. 3,779,970 (Evani I) discloses at column 1, lines 51 to 59 that "It is important to the invention that the esterifying moiety be a monohydroxyl containing nonionic surfactant and that the hydrophobic group of the surfactant be spaced apart from the polymer backbone by a hydrophilic polyethylene oxide (polyoxyethylene) chain having at least about 10 oxyethylene units in the chain. In addition, the nonionic surfactant should have an HLB of at least about 12, preferably 14." It is also disclosed at column 4, lines 23 to 28, that "it is believed that the nature of the hydrophobic group of the surfactant and the distance by which it is separated from the backbone of the polymeric material are important in providing the improved flow and leveling properties as well as thickening power to the latex paint." In addition, U.S. Pat. No. 3,794,608 (Evani II) discloses polymer backbones containing nonionic or anionic hydrophilic comonomers which must be balanced in a particular fashion to produce optimum performance of the thickener polymer, as discussed in Evani II at column 3, lines 17 to 25.
Additional patents disclosing random type polymers containing bound surfactants, or other copolymers having a random configuration of hydrophobic groups, include U.S. Pat. No. 4,167,502 (Lewis et al.); U.S. Pat. No. 4,169,818 (DeMartino); U.S. Pat. No. 4,230,844 (Chang I); U.S. Pat. No. 4,268,641 (Koenig et al.); U.S. Pat. No. 4,138,381 (Chang II); European patent application publication No. 13,836 (Chang III) which discloses a hydrophobe poor environment, by describing at page 10, lines 17-19, that the thickening property of the polymer reaches a maximum as surfactant is added; and European patent application publication No. 11,806 (Sonnebend) which discloses at page 11, lines 7-11 that "it is critical to the performance of these products that they contain an effective amount of an in situ, bound surfactant to control the rheology of the aqueous systems thickened with the solubilized emulsion polymer" and is therefore similar to the Evani patents in relying on total polymeric structures.
Another procedure for arranging hydrophobic groups is disclosed in U.S. Pat. No. 4,079,028 (Emmons et al.). Polyurethane polymers are disclosed having hydrophobic groups capping a hydrophilic polyether polymer backbone. Although it is disclosed at column 7, lines 33 to 41 that the polymers "thicken by an associative mechanism such as micellar or other form of association" Emmons et al. at column 14, lines 14 to 28, disclose that terminal monovalent hydrophobic groups are desired. It is disclosed, at column 14, lines 66-68 that these polymer structures are useable for thickening water alone. Another patent of general interest in this area is U.S. Pat. No. 4,209,333 (Ong et al.) which discloses star-shaped polymers using an ester linkage for hydrophobe attachment, in place of a urethane linkage as described in Emmons et al.
Other patents of interest in this general area include: (1) U.S. Pat. No. 3,970,606 (Field et al.) which discloses random interpolymers of N-vinyllactam or acrylamide, hydrophobic group containing vinyl comonomers and a cationic moiety. The patent suggests in Table V at Columns 7 and 8 that as the content of the hydrophobe containing monomer is varied from 0.8 mole % to 9.1 mole %, the thickening efficiency of the polymer is altered only slightly. (2) U.S. Pat. No. 4,228,277 (Landoll I) discloses water-soluble, substituted cellulose ethers modified with C.sub.10 to C.sub.24 alkyl groups and is of interest in disclosing at column 7, lines 57 to 62, that "the behavior of the modified polymers in exhibiting surface activity, as well as their rheological character, suggest that the long chain modified molecules are aggregated into micelle-like clusters in aqueous solution much as is known to happen in the case of more conventional surfactants." As such Landoll I is disclosing that the entire polymer backbone is involved in micelle formation. Similar to the Particle Bridging Theory, Landoll I discloses at column 8, lines 2 to 5 that "surface activity is also noticeable to a significant degree with latex paints where the long-chain alkyl substituted products show a tendency to adsorb on non-polar latex particles." Landoll I also includes a disclosure at column 2, lines 62 to 65 that a uniform presence of hydrophobic groups is required to achieve thickening. Furthermore, it is disclosed at column 8, lines 6 to 16 that viscosity increases with the addition of free surfactant, which exemplifies hydrophobic poor structures. (3) Another patent of general interest is U.S. Pat. No. 4,304,902 (Landoll II) which discloses random copolymers of ethylene oxide and long chain epoxides.